1. Field of the Invention
The present invention relates to a lead frame baking oven which is used for the pre-baking (preparatory heating) of insulating tapes applied to lead frames or for the baking (heating) of adhesive agents (resin pastes) applied to lead frames in order to secure semiconductor chips to the lead frames.
2. Prior Art
Semiconductor devices are manufactured, for example, in the following manner: As shown in FIGS. 10 and 12, an insulating tape 2 coated with an adhesive agent is applied in advance to the under surface (FIG. 10(b)) or upper surface (FIG. 12(b)) of the leads la of a lead frame 1, and the lead frame 1 is pre-baked in baking ovens so as to remove moisture from the insulating tape 2. Then, as shown in FIGS. 11 and 13, a semiconductor chip 3 is chip-bonded to the under surface (FIG. 11(b) or upper surface (FIG. 13(b)) of the insulating tape 2, and afterward the electrodes 3a of the semiconductor chip 3 and the leads la of the lead frame 1 are connected via wires 4.
As well known to artisans skilled in the art, there are other type of devices. In this case, the die carrying part of a lead frame is coated with an adhesive agent, a semiconductor chip is pressed against the adhesive agent so that the chip is secured to the lead frame (called "chip bonding"), and then the adhesive agent is baked in baking ovens so that the electrodes of the semiconductor chip are afterwards connected to the leads of the lead frame via wires (called "wire bonding").
Conventional baking ovens used for pre-baking the insulating tapes or for baking the adhesive agent as described above have a straight tunnel type structure. In these types of ovens, a feeder for conveying the lead frames is surrounded by a heat insulating material, and heaters for heating the objects are installed in series beneath the lead frame conveying path, and a cooling device for cooling the lead frames is installed above the conveying path.
Baking ovens of this type are described in, for example, Japanese Patent Application Laid-Open (Kokai) Nos. 55-127025, 55-127030 and 56-87331 and in Japanese Patent Application Publication (Kokoku) No. 3-45539. In particular, the Japanese Patent Application Laid-Open (Kokai) No. 55-127025 relates to a tunnel type oven that supplies into the tunnel with inert gas which is lower in the temperature than the eutectic temperature of solder, the Japanese Patent Application Laid-Open (Kokai) No. 55-127030 relates to an oven provided with a heater inside that is movable horizontally and vertically, the Japanese Patent Application Laid-Open (Kokai) No. 56-87331 relates to an oven with sealing plates that are opened by transferred workpieces and prevent supplied gas from escaping, and the Japanese Patent Application Publication (Kokoku) No. 3-45539 relates to an oven provided with a reducing gas supply and a reducing heater.
In the prior art described above, the straight-line structure oven generally has a small lead frame accommodating capacity. As a result, the pre-baking or baking time is short, and the removal of moisture from the insulating tape or the baking of the adhesive agent is not accomplished unless the temperature of the heater is set high. If, however, the pre-baking or baking temperature is set high, there is a danger that the oven's function halts which results in thermal destruction (or deterioration) of the insulating tape, and there is a danger that such a halt of the oven will lead to thermal destruction of the semiconductor chip. In order to prevent the thermal destruction of the insulating tape or semiconductor chip, the insulating tape or semiconductor chit is protected during the halt of the oven by the cooling device (which effects a cooling blast) by lowering the temperature. However, once the temperature has been lowered, considerable time is required to bring the temperature back to the original set value.
In the meantime, the Japanese Patent Application Publication (Kokoku) No. 4-39227 discloses an apparatus that allows automatic adjustment of the width of the guide rails that guide the lead frames so that the guide rails can comply with changes in the width of the lead frames that occurs when the type of the product to be handled is changed. However, it is difficult to use this guide rail width adjustment means in the conventional baking oven described above. In other words, ovens that can heat any type of lead frames (which differ in size in particular) has not yet been practically used.